The invention concerns an antenna configuration for use in a magnetic resonance apparatus, which is designed as a phase-controlled array and comprises at least two individual antennas each of which comprises at least one conductor loop, one tuning network and one matching network, wherein the tuning network contains at least one tuning capacitance and the matching network contains at least one matching capacitance, and the individual antennas are each combined into separate modules which are positioned on and mounted to a support body and can be removed therefrom in a non-destructive fashion.
An antenna configuration of this type is disclosed in U.S. Pat. No. 6,084,411.
The invention concerns an imaging device which utilizes nuclear magnetic resonance (NMR or MR) by using antennas (coils) which are used to transmit and/or receive frequency signals (Larmor frequency) and are combined into arrays (fields), wherein the individual antenna consists of a conductor path that defines a surface.
Nuclear magnetic resonance is widely used, being an important imaging method. It utilizes the effect that nuclear spins are excited in a homogeneous magnetic field (B0) when energy is supplied by means of electromagnetic waves of certain frequencies due to absorption. The frequency is thereby determined by the strength of the constant magnetic field (B0) and the special characteristic properties of the nucleus. After a short time, the excited spins return into their ground state, i.e. a state of lower energy, and emit a radio frequency electromagnetic signal which can be detected by receiving antennas and be used for constructing an image. It is fundamentally possible to use the same antenna elements both for excitation (transmitting) of a signal and for receiving a signal. Several antennas may be combined into so-called arrays. The larger the density, i.e. the number of individual antennas per unit area, the better the signal-to-noise ratio (SNR), which can be utilized e.g. for increasing the resolution of the derived image. One further important advantage of antenna arrays is the possibility of using parallel-imaging methods, e.g. SENSE or GRAPPA. These methods realize a higher recording speed.
The present invention concerns an antenna array consisting of several individual antennas.
In medical imaging using NMR, high-frequency magnetic fields in the MHz range are received from the body of a person or an animal by means of an RF antenna and are further processed for imaging.
A receiving antenna designed as a surface antenna (local antennas or local coils) which is adapted to the surface of the object and the area to be investigated, obtains a higher signal-to-noise ratio (SNR) in the MR image compared to that received by a whole-body antenna.
A smaller surface antenna thereby yields a higher SNR than a larger surface antenna, but also has a correspondingly smaller field-of-view (FOV, imaged area). For this reason, one often uses a plurality of smaller antennas or an array of smaller antennas instead of one individual large antenna for investigating a larger FOV. Each individual receiving antenna thereby requires its own receiving path which consists of preamplifier, cable and receiver. Devices of this type are called phased array antennas or antenna arrays.
The individual antennas are thereby generally disposed on a surface which is adapted to the geometry of the area to be investigated.
The antenna arrays have one problem due to the fact that, when several individual antennas are arranged next to one another, it may happen that a high-frequency current in one of the individual antennas induces a voltage in neighboring individual antennas. This is called coupling of antennas with respect to one another. Couplings occur both with configurations of circular polarized antennas and also with configurations of linearly polarized individual antennas. Couplings deteriorate the signal-to-noise ratio (SNR) and generate artefacts in the MR image. Examination of coupled individual antennas is moreover more demanding than examination of individual antennas that are not coupled. One aim of the design of phased array antennas is therefore to avoid coupling of individual antennas, if possible.
Decoupling Techniques
An antenna array of the above-mentioned type is described in U.S. Pat. No. 4,825,162 [1]. The antenna array comprises several individual antennas that are disposed next to one another. For decoupling, neighboring individual antennas partially overlap. This overlap reduces the mutual inductance of neighboring individual antennas. The overlap also requires that the antenna conductors cross each other, thereby generating corresponding crossing points. The antenna conductors must be guided such that they are insulated from each other at the crossing points. At higher frequencies, the capacitances formed at the crossing points moreover generate, in turn, capacitive couplings.
The above-cited document U.S. Pat. No. 4,825,162 [1] mentions a further measure to reduce couplings. It consists in selecting the impedance of a preamplifier which is connected to the individual antenna in such a fashion that an impedance, acting on the connectors of the individual antenna, which is also determined by the input resistance of the preamplifier, has a maximum value. In consequence thereof, the current induced in the individual antennas almost vanishes, whereby the voltage induced in neighboring individual antennas becomes correspondingly small and negligible. However, the expense is great for obtaining sufficient decoupling in this fashion. This type of decoupling is therefore used in practice together with other decoupling techniques. This type of decoupling is referred to as preamplifier decoupling below.
A further antenna array is described in U.S. Pat. No. 5,216,368 [3]. The antenna array comprises two antenna systems that are aligned perpendicularly with respect to one another. When these two antenna systems are exactly aligned, they are decoupled from each other solely due to their arrangement. Asymmetries, however, cause couplings of the two antenna systems which are compensated for by a capacitor which connects the two antenna systems.
DE 41 13 120 C2 [4] describes an antenna system with a standing wave trap. The standing wave trap suppresses undesired radio frequency coupling via the lines of the antenna.
DE 10 2004 046 188 A1 [5] describes an antenna array for a magnetic resonance apparatus with capacitive compensation of the inductive coupling. Neighboring individual antennas each have an interruption. The individual antennas are electrically connected in parallel at the interruptions. At least one of the interruptions is bridged by a capacitive element, wherein the capacitive element has a capacitance value at which the individual antennas are decoupled from each other. However, the individual antennas are galvanically connected via the decoupling circuit, which generates a common mode signal connection.
DE 102 44 172 A1 [6] describes a further decoupling element consisting of a galvanically contact-free decoupling antenna. This decoupling antenna is designed and/or arranged in such a fashion that it inductively couples with the neighboring individual antennas in such a fashion that the inductive coupling between the two relevant individual antennas is minimum. For this reason, one often completely omits decoupling elements in antennas of this type and the individual antennas are only decoupled from each other via the above-described preamplifier decoupling.
Conform Antennas
Flexible antennas are advantageously used for arranging the individual antennas on an area which is optimally adapted to the geometry of the area to be investigated.
U.S. Pat. No. 6,650,926 B1 [7] describes e.g. a phased array antenna configuration consisting of several rigid antenna elements which are connected to each other via a hinge and can thereby be adjusted to the geometry of the patient.
In contrast thereto, US 2008/0238424 A1 [8] describes a phased array antenna configuration, in which the entire RF antenna is mounted to a flexible substrate and is therefore flexible as a whole.
Another possibility of adjusting a phased array antenna to the geometry of the object to be imaged consists in forming the phased array antenna in a modular fashion such that the individual antennas of the phased array antenna can be arranged in correspondence with the desired geometry. An antenna system of this type is described in U.S. Pat. No. 6,084,411 [9].
All of these approaches have one common problem. Shaping of an antenna array or relocating individual antennas of an antenna array changes the mutual coupling of the individual antennas, in consequence of which the decoupling elements must be readjusted for minimizing the coupling of the individual antennas.
It is the underlying object of the device in accordance with the invention to define individual antenna modules, which can be arranged around the measuring volume in a simple fashion, which are also electromagnetically decoupled from each other, and which can preferably be positioned close to the measuring volume in order to ensure that the received MRI image has the highest possible signal-to-noise ratio.